Power storage devices that are well known include, for example, secondary batteries, such as lithium ion batteries, nickel hydrogen batteries, and lead batteries, and electrochemical capacitors, such as electric double layer capacitors. Due to downsizing of cellular phones or limited spaces for installation, and other reasons, smaller power storage devices are sought. Thus, lithium ion batteries with high energy density are attracting attention. Metal cans have been used as packaging materials for lithium ion batteries. However, there has been a growing trend to use multi-layer films (e.g. a film having a structure of base material layer/metal foil layer/sealant layer) which are lightweight, have high heat dissipation, and can be produced at low cost.
Lithium ion batteries using the multi-layer film as a packaging material are so configured that the packaging material including an aluminum foil layer as a metal foil layer contains the battery contents to prevent moisture from penetrating into the battery. A lithium ion battery adopting such a configuration is referred to as an aluminum laminated film type lithium ion battery. The battery contents of the lithium ion battery include a positive electrode, a negative electrode, and a separator, and an electrolytic solution or an electrolyte layer. The electrolytic solution contains an aprotic solvent, such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate, having osmotic force, and lithium salt as an electrolyte dissolved in the aprotic solvent. The electrolyte layer is formed of a polymer gel impregnated with the electrolytic solution.
As the aluminum laminated film type lithium ion battery, there is known, for example, an embossed lithium ion battery obtained by forming a recess on a part of a packaging material by cold forming, accommodating battery contents in the recess, and folding back the rest of the part of the packaging material to seal the edge portions by heat sealing. The packaging material configuring such a lithium ion battery is required to show stable sealing performance when heat-sealed and to resist degradation of the lamination strength between the aluminum foil layer and the sealant layer due to the electrolytic solution of the battery contents.
In this regard, PTL 1, for example, proposes a packaging material which is provided with a heat sealing layer (sealant layer) including an adhesive polymethylpentene layer.
The energy density of the lithium ion battery can be made higher by making the depth of the recess formed by cold forming deeper. However, when the recess is deeper, micro cracks are prone to occur in the sealant layer due to strain caused during cold forming. Specifically, whitening is likely to occur in the drawn portions, such as shaped surfaces and corners, of the sealant layer. Whitening caused by cold forming reduces insulation properties and thus accelerates degradation of battery performance. Therefore, reducing whitening due to cracks and bending is desired.
In this regard, PTL 2, for example, proposes a packaging material exhibiting stable sealing performance, heat resistance, insulation properties, and formability, and including a heat sealing layer (sealant layer) formed of a polypropylene layer with a high melting point of 150° C. or more, and a propylene-ethylene random copolymer layer.